Question

Students should always be careful of the chemical reaction between jewelry and laboratory reagents. The table below shows the reduction reactions and potentials for some common lab reagents and metals.

Ox + ne1-	Red	Eo(V)
Ni2+(aq) + 2e1-	↔ Ni(s)	-0.23
SO42-(aq) + 4H1+(aq) + 2e1-	↔H2SO3(aq) + H2O	+0.20
Cu2+(aq) + 2e1-	↔ Cu(s)	+0.34
Ag1+(aq) + e1-	↔ Ag(s)	+0.80
Pt2+(aq) + 2e1-	↔ Pt(s)	+1.19
Cr2O72-(aq) + 14H1+(aq) + 6e1-	↔ 2Cr3+(aq) + 7H2O	+1.33
Au3+(aq) + 3e1-	↔ Au(s)	+1.50
MnO41-(aq) + 8H1+(aq) + 5e1-	↔ Mn2+(aq) + 4H2O	+1.51

(a) What is the net redox reaction that occurs when Au comes into contact with acidic KMnO₄? (Use the lowest possible coefficients. Omit states-of-matter from your answer.)

(b) What is E°_cell for the reaction?

________ V

(c) Which metal is the least reactive?

Ag

Au   

Cu

Ni

Pt

(d) Which reagent is the most reactive?

Cr₂O₇²⁻

MnO₄⁻   

SO₄²⁻

Answer 1

a)

oxidation reaction (Anode)

Au - 3e  $\to$ Au³⁺ (1) E_red⁰ = + 1.50 V

reduction reaction (Cathode)

MnO₄^- + 8H⁺ + 5e  $\to$ Mn²⁺ + 4 H₂O (2) E⁰_red = + 1.51 V  

Eq. 1 *5 + Eq.2*3

Balanced reaction is

5 Au + 3 MnO₄^- + 24 H⁺$\to$ 3 Mn²⁺ + 5 Au³⁺ + 12 H₂O

b)

E⁰_cell = E⁰_cathode - E⁰_anode = 1.51 - 1.50 = +  0.01 V

C)

Least reactive metal has highest value of standard reduction potential.

Au has highest value standard reduction potential among the given metals . So Au is least reactive.

d)

Standard reduction potential of MnO₄^- is more positive than Cr₂O₇^2- and SO₄^2- hence MnO₄^- is stronger oxidizing agent hence most reactive.